This invention relates to modulation systems and more particularly to modulation systems in which the phase shift keying of vectors is used for the transmission of data.
Conventional polyphase modulating circuits employing balanced mixers as the phase change element usually consist of input and output transformers cross-coupled by the usual diode quad arrangement using four diodes which change the phase of the input carrier to produce the output vector at 180.degree. or 0.degree. depending upon polarity of the data signal applied to the diodes. This represents one pair of output vectors. When a plurality of vectors are required such as in the transmission of data in which, for example, eight vectors are required to transmit three parallel data bits or streams, additional balanced mixers are required for each pair of vectors transmitted. Such conventional polyphase modulating circuits employing balanced mixers as the phase change element rely on perfect balance of all the phase change elements to achieve exact 180.degree. changes in output for each mixer. In practice, the best that is usually realized is in the order of one degree phase accuracy. This is due to imperfect balance between individual diodes of the diode quad arrangement and transformer imbalance. This leads to a phase error in one of the vector pairs which is not adjustable and which results in system performance degradation. For example, in polyphase devices such as an eight-phase modulator which generates eight independent modulation vectors, additional mixers must be used as amplitude modulators to derive the conventional two-level four-phase vectors that ultimately produce the required eight vector states. These mixers are very sensitive to bias current fluctuation due to temperature or supply voltage variations, when used in a quasi-linear amplitude control mode. The eight vectors when derived in this way, from a two-level four-phase arrangement, or by way of orthogonal amplitude modulators, do not have a primary adjustment feature and alignment of the eight vector states becomes a complex iterative adjustment with all vectors inter-related; that is, when one vector is adjusted for correct phase and amplitude, for example by diode bias current control, the adjusted vector changes the adjustment of other vectors often resulting in a phase error of as much as one or two degrees. At high bit rates of the input data stream, for example three bits per Hertz of the channel bandwidth, such errors become unacceptable within a given channel bandwidth. That is, the narrower the channel bandwidth, the more critical the phase accuracy of individual transmitted vectors become. Such phase error results in unequal angular separation between associated vector pairs and produces phase distortion resulting in system errors in the phase detection process of associated receiving equipment.